Nineteen-year-old Jared James Abrahams of Temecula entered his in federal court in Orange County.
Abrahams, a computer science student, now faces up to 11 years in prison and fines of up to $1 million.
His sentencing is scheduled for March.
Abrahams is also accused of emailing the victims and threatening to publish images of them on social media sites if they didn't send him additional pictures or undress for him.
He told the judge he has autism and has been under treatment for it for the past 10 years.
Abrahams agreed to plead guilty last month to three counts of extortion and one count of unauthorized access of a computer.
The Associated Press contributed to this report.